NO Decomposition on Rh Supported CNTs

It is well known that NO is one of the most predominant pollutants. It is the main source of acid rain and has strong carcinogenicity. Therefore, removal of NO is one of the key environmental subjects in the world nowadays. With the discovery of carbon nanotubes and their large-scale synthesis1-2, attention is now being focused on their potential application in various fields of materials research. In the field of heterogeneous catalysis, numerous carbon materials have been used to disperse …     

催化裂解CH4制备碳纳米管的影响因素

以柠檬酸法制备的Fe-MgO、Co-MgO和Ni-MgO为催化剂,CH4为碳源气,H2为还原气,在873、973和1073 K制备出碳纳米管,通过TEM和拉曼光谱表征,讨论了催化剂、制备温度、反应时间等因素对碳纳米管形貌、产率和内部结构的影响.结果表明:不同的催化剂在相同的温度下制备的碳纳米管的形态和内部结构有很大的差异.其中Fe-MgO催化剂制备的碳纳米管管径粗,且大小不均匀,而Ni-MgO催化剂制备的碳纳米管管径较细、较均匀.碳纳米管的产率随着裂解温度的变化而改变.Fe-MgO催化剂制备碳纳米管的产率随制备温度的升高而提高,而Ni-MgO催化剂制备碳纳米管的产率随制备温度的升高而降低.Fe-MgO催化剂制备碳纳米管,在1073K甚至更高的制备温度才能达到其最高产率.Co-MgO催化剂制备碳纳米管的产率在973 K左右产率较高,而用Ni-MgO催化剂制备碳纳米管,则在873 K甚至更低的制备温度就能达到最高产率.反应时间与碳纳米管的产率不成正比,有一最佳反应时间,如Ni-MgO催化剂的最佳反应时间为2 h.     

Carbonization of electrospun poly(acrylonitrile) nanofibers containing multiwalled carbon nanotubes observed by transmission electron microscope with in situ heating

Composite nanofibers with 5% w/w multiwalled carbon nanotubes (MWCNTs) in polyacrylonitrile (PAN) were fabricated using the electrospinning technique. Morphological development during the carbonization process was characterized by transmission electron microscopy (TEM) with in situ heating. It was found that the orientation of graphitic layers increases with temperature and does not change significantly with time during our TEM measurement, except the 750 °C. In the heating stage at 750 °C noticeable enhancement of orientation with time was observed. The presence of embedded CNTs enhances the order of the formed graphitic structures even when the CNTs are irregular or entangled. The results indicate that embedded MWCNTs in the PAN nanofibers nucleate the growth of carbon crystals during PAN carbonization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Formation of carbon nanotubes from a silicon carbide/carbon composite

The reaction of a SiC/C composite powder in an arcing plasma forms carbon nanotubes in good yield. Besides carbon nanotubes, a Si/C composite composed of β SiC covered with a shell of graphite is formed. The graphitic carbon surface layers of the carbon shell of this composite reacts further to form carbon nanotubes when heated to 600 °C. This process seems highly effective since only a small overall low weight loss, indicative for a complete carbon shell oxidation is observed by thermal analysis. The formation of the carbon nanotubes from SiC is unlikely since no SiO₂ has been found when heating the SiC/C core shell composite to its reaction temperature of 600 °C under O₂. The CNTs formed are of good quality with 3 to 6 concentric walls and high aspect ratio. Occasionally even single walled carbon naotubes have been observed.     

One-step hydrothermal synthesis of flower-like SnO2/carbon nanotubes composite and its electrochemical properties

In this work, flower-like SnO₂/carbon nanotubes (CNTs) composite was synthesized by one-step hydrothermal method for high-capacity lithium storage. The microstructures of products were characterized by XRD, FESEM and TEM. The electrochemical performance of the flower-like SnO₂/CNTs composite was measured by cyclic voltammetry and galvanostatic charge/discharge cycling. The results show that the flower-like SnO₂/CNTs composite displays superior Li-battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1,230 and 842 mAh g^?1, respectively. After 40 cycles, the reversible discharge capacity is still maintained at 577 mAh g^?1 at the current densities of 50, 100 and 500 mA g^?1, indicating that it’s a promising anode material for high performance lithium-ion batteries.     

Simple and Precise Quantification of Iron Catalyst Content in Carbon Nanotubes Using UV/Visible Spectroscopy

Iron catalysts have been used widely for the mass production of carbon nanotubes (CNTs) with high yield. In this study, UV/visible spectroscopy was used to determine the Fe catalyst content in CNTs using a colorimetric technique. Fe ions in solution form red–orange complexes with 1,10-phenanthroline, producing an absorption peak at λ=510 nm, the intensity of which is proportional to the solution Fe concentration. A series of standard Fe solutions were formulated to establish the relationship between optical absorbance and Fe concentration. Many Fe catalysts were microscopically observed to be encased by graphitic layers, thus preventing their extraction. Fe catalyst dissolution from CNTs was investigated with various single and mixed acids, and Fe concentration was found to be highest with CNTs being held at reflux in HClO₄/HNO₃ and H₂SO₄/HNO₃ mixtures. This novel colorimetric method to measure Fe concentrations by UV/Vis spectroscopy was validated by inductively coupled plasma optical emission spectroscopy, indicating its reliability and applicability to asses Fe content in CNTs.     

Quantitative interpretation to the chain mechanism of free radical reactions in cyclohexane pyrolysis

Pyrolysis of cyclohexane was conducted with a plug flow tube reactor in the temperature range of 873-973 K. Based on the experimental data, the mechanism and kinetic model of cyclohexane pyrolysis reaction were proposed. The kinetic analysis shows that overall conversion of cyclohexane is a first order reaction, of which the rate constant increased from 0.0086 to 0.0225 to 0.0623 s-1 with the increase of temperature from 873 to 923 to 973 K, and the apparent activation energy was determined to be 155.0±1.0 kJ mol-1. The mechanism suggests that the cyclohexane is consumed by four processes:the homolysis of C-C bond (Path I), the homolysis of C-H bond (Path II) in reaction chain initia- tion, the H-abstraction of various radicals from the feed molecules in reaction chain propagation (Path III), and the process associated with coke formation (Path IV). The reaction path probability (RPP) ratio of XPath I:XPath II : XPath III : XPath IV was 0.5420:0.0045:0.3897:0.0638 at 873 K, and 0.4336 : 0.0061 : 0.4885 : 0.0718 at 973 K, respectively.     

TEM investigation of TiNx-PACVD-coatings

The wear resistance of cermet cutting tools can be remarkably increased by TiN_x coatings. These layers are deposited at substrate temperatures of 723 K, 773 K and 973 K using a plasma-assisted chemical vapour deposition (PACVD) process. TEM investigations combined with EDXS analysis and electron diffraction gave information on structure and composition of the TiN_x layers and the interface range. X-ray structure investigations were performed additionally.The structure and the chlorine content of layers and interfaces change in dependence on the deposition temperature. All coatings show a columnar structure, but the fibre diameter increases with temperature. The fine-grained TiN_x layer deposited at 723 K has the highest chlorine content, a low-developed columnar structure and a 111 texture. The coatings deposited at 773 K and 973 K contain less chlorine impurities and have a 100 preferred orientation. The fibre structures at 723 K and 773 K can be resolved into single crystallites. By TEM investigations the fibres formed at 773 K are proved to be an accumulation of neighbouring and similarly oriented crystallites. Grain size determined by X-ray analysis and fibre diameter agree with each other. Grain sizes determined more exactly from TEM images are 6 nm at 723 K, 10 nm at 773 K and 30 nm at 973 K. In the interface region the thickness and the chlorine content of this zone decreases with increasing deposition temperature and simultaneously the layer adhesion increases.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Synthesis and electrochemical performance of CeO2@CNTs/S composite cathode for Li–S batteries

The shuttle effect of lithium-sulfur (Li–S) battery is one of the crucial factors restraining its commercial application, because LiPSs (lithium polysulfides) usually leads to poor cycle life and low coulomb efficiency. Some studies have shown that metal oxides can adsorb soluble polysulfides. Herein, CeO₂ (cerium-oxide)-doped carbon nanotubes (CeO₂@CNTs) were prepared by the hydrothermal method. The polar metal oxide CeO₂ enhanced the chemisorption of the cathode to LiPSs and promoted the redox reaction of the cathode through catalysis properties. Meanwhile, the carbon nanotubes (CNTs) enhanced cathode conductivity and achieved more sulfur loading. The strategy could alleviate polysulfide shuttling and accelerate redox kinetics, improving Li–S batteries' electrochemical performances. As a result, the CeO₂@CNTs/S composite cathode showed the excellent capacity of 1437.6 mAh g⁻¹ in the current density of 167.5 mA g⁻¹ at 0.1 C, as well as a long-term cyclability with an inferior capacity decay of 0.17% per cycle and a superhigh coulombic efficiency of 100.434% within 300 cycles. The superior electrochemical performance was attributed to the polar adsorption of CeO₂ on polysulfides and the excellent conductivity of CNTs.

     

Effect of oxygen-containing functional groups of carbon materials on the performance of Li–O2 batteries

In this communication, we present some new findings on surface oxidized carbon nanotubes (CNTs) when used as cathode of Li–O₂ batteries. It is found that the content of oxygen-containing functional groups has a significant influence on the electrochemical performance of Li–O₂ batteries, by altering the electrical conductivity and density of electrocatalytically active sites of the CNTs and promoting side reactions of the electrolyte. An optimal surface oxygen atomic content of 6.0 at.% on CNTs is found to reach a balance and give the best cycling stability of the Li–O₂ battery under constant capacity and constant current density tests.     

Pseudocapacitive behaviors of nanostructured manganese dioxide/carbon nanotubes composite electrodes in mild aqueous electrolytes: effects of electrolytes and current collectors

Nanostructured MnO₂/carbon nanotubes composite electrode material was prepared using the liquid-phase deposition reaction starting with potassium permanganate (KMnO₄) and manganese acetate (Mn(Ac)₂·4H₂O) as the reactants and carbon nanotubes (CNTs) as the substrates. The structure and morphology of the material was characterized by X-ray diffraction, infrared spectroscopy, and transmission electron microscope techniques. The electrochemical properties of the nano-MnO₂/CNTs composite electrode in 1 M LiAc and 1 M MgSO₄ solutions and in 1 M RAc (R = Li, Na, and K)–1 M MgSO₄ mixed solutions, respectively, were studied. Experimental results demonstrated that the specific capacitance and rate discharge ability of the nano-MnO₂/CNTs composite electrode in 1 M LiAc–1 M MgSO₄ mixed solution is superior to that in 1 M LiAc or 1 M MgSO₄ solution. For the 1 M RAc (R = Li, Na, and K)–1 M MgSO₄ mixed electrolytes, the specific capacitance of the composite electrode was found to be in the following order: LiAc > NaAc > KAc.     

Electrochemical lithiation and de-lithiation of carbon nanotube-Sn2Sb nanocomposites

Nanocomposites of carbon nanotubes (CNTs) with Sn₂Sb alloy nanoparticles were prepared by KBH₄ reduction of SnCl₂ and SbCl₃ precursors in the presence of CNTs. SEM and TEM examinations showed that most of the Sn–Sb alloy nanoparticles were present in high dispersion in the CNT web, while others were deposited directly on the outside surface of the carbon nanotubes. Constant current charge and discharge tests using the nanocomposites as Li⁺ storage compounds showed higher specific capacities than pristine CNTs and better cyclability than unsupported Sn₂Sb particles. The first cycle de-lithiation capacity of 580 mAh/g from a CNT–56 wt%Sn₂Sb nanocomposite was nevertheless reduced to 372 mAh/g after 80 deep charge and discharge cycles. The uniform dispersion of Sn₂Sb alloy in the CNT web and on the surface of CNTs have substantially improved the usability of the Sn₂Sb particles to the extent that the nanocomposites of CNTs and Sn₂Sb may be considered as a candidate anode material for Li-ion batteries.     

Microwave Synthesized In2S3@CNTs with Excellent Properties inLithium‐Ion Battery and Electromagnetic Wave Absorption

The three‐dimensional nanoflower‐like β‐In₂S₃ composited with carbon nanotubes (CNTs) has been synthesized by a single mode microwave‐assisted hydrothermal technique. The In₂S₃ and CNTs nanocomposites (In₂S₃@CNTs) were investigated as the anode materials of lithium batteries (LIBs) and the electromagnetic wave absorption materials. For LIBs applications, the In₂S₃@CNTs nanocomposite exhibited excellent cycling stability with a high reversible charge capacity of 575 mA⋅h⋅g^–1 after 300 cycles at 0.5 A⋅g^–1. In addition, the In₂S₃@CNTs used as electromagnetic wave absorber displayed a maximum reflection loss of –42.75 dB at 11.96 GHz with a thickness of 1.55 mm.     

Enhanced stability of Pt electrocatalysts by nitrogen doping in CNTs for PEM fuel cells

Electrochemical stabilities of Pt deposited on carbon nanotubes (CNTs) and nitrogen-doped carbon nanotubes (CN_x) of different nitrogen contents are compared with accelerated durability tests (ADT) for the first time. Transmission electron microscopy (TEM) images reveal the different structures of CNTs and CN_x, and the decrease of Pt particle size with the nitrogen incorporation into CNTs. Based on the loss of electrochemical surface area (ESA) and TEM images, Pt/CN_x exhibited much higher stability than Pt/CNTs, and the Pt stability increases with the increase of nitrogen contents in the CN_x supports.     

The study of carbon nanotubes as conductive additives of cathode in lithium ion batteries

Carbon nanotubes (CNTs), including multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs), are employed as conductive additives in lithium ion batteries. The effects of MWCNTs’ carbon precursors, diameter, and weight fraction on the electrochemical behavior of MWCNTs/LiCoO₂ composite cathode are investigated. Meanwhile, a comparison is made between SWCNTs /LiCoO₂ and MWCNTs/LiCoO₂. Among the three kinds of carbon precursors: CH₄, natural gas, and C₂H₂, MWCNTs prepared from CH₄ are very fit for acting as conductive additives due to their better crystallinity and lower electrical resistance. MWCNTs with smaller diameter favor improving the electrochemical behavior of MWCNTs/LiCoO₂ composite cathode at higher charge/discharge rate owing to their advantage in primary particle number in unit mass. To make full use of LiCoO₂ at higher rate, it is necessary to add at least 5 wt.% of MWCNTs with a diameter 10~30 nm. However, SWCNTs are not expected to be added into LiCoO₂ composite cathode since they tend to form bundles.     

Synthesis,Crystal Structure and Lithium Motion of Li8SeN2 and Li8TeN2

The compounds Li₈EN₂ with E = Se, Te were obtained in form of orange microcrystalline powders from reactions of Li₂E with Li₃N. Single crystal growth of Li₈SeN₂ additionally succeeded from excess lithium. The crystal structures were refined using single‐crystal X‐ray diffraction as well as X‐ray and neutron powder diffraction data (I4₁md, No. 109, Z = 4, Se: a = 7.048(1) Å, c = 9.995(1) Å, Te: a = 7.217(1) Å, c = 10.284(1) Å). Both compounds crystallize as isotypes with an anionic substructure motif known from cubic Laves phases and lithium distributed over four crystallographic sites in the void space of the anionic framework. Neutron powder diffraction pattern recorded in the temperature range from 3 K to 300 K and X‐ray diffraction patterns using synchrotron radiation taken from 300 K to 1000 K reveal the structural stability of both compounds in the studied temperature range until decomposition. Motional processes of lithium atoms in the title compounds were revealed by temperature dependent NMR spectroscopic investigations. Those are indicated by significant changes of the ⁷Li NMR signals. Lithium motion starts for Li₈SeN₂ above 150 K whereas it is already present in Li₈TeN₂ at this temperature. Quantum mechanical calculations of NMR spectroscopic parameters reveal clearly different environments of the lithium atoms determined by the electric field gradient, which are sensitive to the anisotropy of charge distribution at the nuclear sites. With respect to an increasing coordination number according to 2 + 1, 3, 3 + 1, and 4 for Li(3), Li(4), Li(2), and Li(1), respectively, the values of the electric field gradients decrease. Different environments of lithium predicted by quantum mechanical calculations are confirmed by ⁷Li NMR frequency sweep experiments at low temperatures.     

The effect of temperature and oxygen partial pressure on the reduction mechanism in the Pr2CuO4/Ce0.9Gd0.1O1.95 system

The electrochemical behavior of a porous electrode based on Pr₂CuO₄ (PCO) screen printed on the surface of Ce_0.9Gd_0.1O_1.95 (CGO) solid electrolyte is studied by impedance spectroscopy. The rate-determining stages of the oxygen reduction reaction at the PCO/CGO interface are found for the oxygen partial pressure interval of 30–10₅ Pa and temperatures of 773–1173 K. Changeover of the rate-determining stage of electrode reaction is shown to occur depending on the temperature and the oxygen partial pressure. The PCO electrode polarization resistance is 1.7 Ω cm² at 973 K in air and remains constant at thermocycling of the electrochemical cell in the temperature range of 773–1173 K. Based on the found data, PCO can be considered as the promising cathodic material for solid-oxide fuel cells operating at moderate temperatures (773–973 K).     

Nitrogen‐Doped Carbon Nanomaterials: Synthesis,Characteristics and Applications

Nonmetallic carbon‐based nanomaterials (CNMs) are important in various potential applications, especially after the emergence of graphene and carbon nanotubes, which demonstrate outstanding properties arising from their unique nanostructures. The pristine graphitic structure of CNMs consists of sp² hybrid C?C bonds and is considered to be neutral in nature with low wettability and poor reactivity. To improve its compatibility with other materials and, hence, for greater applicability, CNMs are generally required to be functionalized effectively and/or doped with heteroatoms in their graphitic frameworks for feasible interfacial interactions. Among the various possible functional/doping elements, nitrogen (N) atoms have received much attention given their potential to fine tune the intrinsic properties, such as the work‐function, charge carrier concentration, surface energy, and polarization, of CNMs. N‐doping improves the surface energy and reactivity with enhanced charge polarization and minimal damage to carbon frameworks. The modified surface energy and chemical activity of N‐doped carbon nanomaterials (NCNMs) can be useful for a broad range of applications, including fuel cells, solar cells, Li‐ion batteries, supercapacitors, chemical catalysts, catalyst supports, and so forth.     

共聚物模板辅助合成高倍率性能多孔Li2FeSiO4@C/CNTs纳米复合正极材料

通过溶胶-凝胶法制备了Li₂FeSiO₄@C/CNTs(LFS@C/CNTs)纳米复合材料,其中三嵌段共聚物P123用作结构导向剂和碳源,碳纳米管作为导电线提高材料的导电性。LFS@C/CNTs不仅具有海绵状纳米孔,能够与电解液充分接触改善锂离子的传输路径,同时由非晶碳和碳纳米管构成的三维桥联导电网络利于电子的快速传递,提高了材料大电流充放电能力和循环稳定性。复合后的LFS@C/CNTs的高倍率性能相比LFS@C明显提高, 当CNTs的掺量为4%,电压窗口为1.5~4.5 V,0.1C电流密度下放电比容量为182 mAh·g^-1。在10C经70次循环后该材料的放电比容量能保持在117 mAh·g^-1,是LFS@C放电比容量(55 mAh·g^-1)的两倍。     

Mössbauer study of the thermal decomposition of alkali dihydroxo tetrapropionato ferrates(III)

Thermal decomposition of alkali dihydroxo tetrapropionato ferrates(III), M₃[Fe(C₂H₅COO)₄(OH)₂]xH₂O (M=Li, Na, K) has been studied upto 973 K. The complexes were calcined isothermally at various temperatures i. e., 473, 573, 773 and 973 K. The intermediates/products have been characterized by Mössbauer, infrared spectroscopies and XRD powder diffraction. The anhydrous complexes directly decompose to give -Fe₂O₃ and alkali metal carbonate without undergoing reduction to iron(II) moiety. An increase in the particle size and internal magnetic field of -Fe₂O₃ has been observed with increasing decomposition temperature. At higher temperature (973 K) MFeO₂ is formed as the final thermolysis product due to a solid state reaction between -Fe₂O₃ and alkali metal carbonate.